The human knee joint contains two tibio-femoral pairs of articular surfaces, namely a medial pair and a lateral pair of articular surfaces, which are respectively formed by a femoral condyle and a cup-shaped tibial or meniscal articular surface interacting with it. In knee prostheses, these anatomical conditions are simulated by two femoral, condyle-shaped articular surfaces and two cup-shaped articular surfaces on the tibia side. The lateral distance between the femoral articular surfaces must in this case correspond exactly to the distance between the tibial articular surfaces. To ensure this, the femoral articular surfaces on the one hand and the tibial articular surfaces on the other hand are generally formed in a single piece (U.S. Pat. No. 4,081,866; WO/79/007339, FIG. 36). Although it is also known, WO/79/007339, FIG. 34) to use for the replacement of the femoral articular surfaces femorally unconnected single prostheses which interact with cup-shaped tibial articular surfaces connected in a single piece, this is rejected nowadays because it is not ensured that the distance between and alignment of the femoral sliding surfaces match the tibial part of the prosthesis.
Similarly regarded as outmoded is a prosthesis technique in which unconnected femoral and tibial parts of the prosthesis are implanted by means of one and the same positioning instrument, which ensures the same distance between the sliding surfaces on both sides (U.S. Pat. No. 3,949,428). Furthermore, it is known in those cases in which replacement of the articular surfaces takes place only on one side (femoral or tibial) or where an exact alignment of the interacting articular surfaces faces does not matter, to implant the unconnected parts of the prosthesis by means of a positioning instrument which ensures that they are parallel (EP-A 824 904; W. Link: brochure xe2x80x9cSchalen-Kniegelenk-prothesen-System SKIxe2x80x9d [cup knee joint prosthesis system SKI]).
To sum up, it can be stated that known modern knee prosthesis systems which provide a femoral articular surface replacement and tibial articular surface replacement as total prostheses and in which the tibial sliding surfaces formed in a cup-shaped manner are designed to fit exactly with the femoral sliding surfaces only ever have femoral sliding surfaces connected in a single piece. One of the reasons why this configuration has established itself is that it offers the possibility of forming a sliding surface for the patella, or a patella replacement, which connects the parts of the femoral sliding surfaces as a patella plate (U.S. Pat. No. 4,081,866). Since prostheses of this type have proved successful, they are often also used when the femoral patella slideway is still well preserved and actually need not be replaced.
The invention requires a knee prosthesis system which comprises a single-piece tibia plateau with two sliding cavities and a single-piece femur component with two condylar sliding surfaces, the distance between and shape of which are adapted to the tibial sliding cavities. It additionally creates an implantation alternative for those cases in which the femoral patella sliding surface is still so well preserved that it need not be replaced, in that it adds to this system, as an alternative to the single-piece femur component, a pair of connected femoral condylar cups, the shape of the sliding surfaces of which is similarly adapted to the tibial sliding cavities like the condylar sliding surfaces of the single-piece femur component. In order that they also assume precisely that position in relation to each other that the condyles of the single-piece component have in relation to each other, the system also includes a positioning instrument for the condylar cups, which gives them a relative position adapted to the tibial sliding cavities.
The invention makes it possible to use one and the same knee prosthesis system for different injuries. If the entire sliding surfaces of the replacement so require, the single-piece condyle component is used. If, on the other hand, the patella sliding surfaces are still in a good condition, only the condylar sliding surfaces are replaced. Moreover, it is possible by a comparatively minor intervention to replace the condylar cups by a single-piece femur component if it should be found over time that the patella sliding surfaces need replacing as well. Finally, an intraoperative change of the initially provided, less radical variant to the more radical variant is also possible.
At the same time, it is not necessary to dispense with complicated shapes of sliding surfaces, as have been developed in the case of single-piece prostheses. This is so because it has so far been customary to design separate prosthesis cups and/or the associated tibia sliding surfaces in such a way that they can tolerate implantation inaccuracies, because their shapes need not be exactly matched to one another. However, it is possible within the scope of the system according to the invention, by the addition of the positioning instrument, similarly to shape the separate prosthesis cups such that they are adapted to the tibial component, like the corresponding condylar parts of the single-piece femur component, because a sufficiently precise implantation is ensured. The shape of and distance between the sliding surfaces of the condylar cups can therefore coincide with the shape of and distance between the condylar sliding surfaces of the single-piece femur component.
Furthermore, it is expedient if the condylar resection surfaces which are required for the positioning of the condylar cups coincide with the corresponding resection surfaces of the single-piece femur component. This not only has the advantage that the same sawing gage can be used. Rather, it also makes it easier to change over from one variant of the femoral prosthesis to the other.
Since the single-piece femur component extends over the entire width of the femur condyles, the associated sawing cuts are to be made not only in the region of the condyles but also in the intermediate region. The latter is not required when using condylar cups. It may even be harmful ifxe2x80x94depending on the position of the resection cutsxe2x80x94part of the femoral sliding surface region of the knee cap could also be affected. It is therefore provided according to the invention that the sawing gage which is used for both variants of femur prostheses has in the intercondylar region a blocking lug which allows the operating surgeon to spare the intercondylar region or even restrict the condylar cuts to the mutually facing sides in exact accordance with the intended position of the edges of the condylar cups.
The restrictions of the condylar cups may be chosen to take any desired form. However, it is expedient if they face mutually parallel, straight side surfaces. This is because their exact alignment with the positioning instrument is then easier, if the latter has matching parallel connecting surfaces.
The prosthesis system according to the invention normally includes a number of size stages with different distances between the condylar cups. The invention correspondingly provides that the mutual distance between the parallel connecting surfaces of the positioning instrument can be set differently in stages. Their adjustability is expediently based on the use of exchangeable parts, which are in each case assigned only to one stage because this reduces the risk of erroneously setting the distance between the condylar cups. A preferred embodiment of the invention provides that the connecting surfaces of the positioning instrumentxe2x80x94whether they are parallel or notxe2x80x94are formed by in each case a fixed jaw of individual clamps which are respectively assigned to a condylar cup and for which, according to a further feature, different mounts assigned to the individual size stages are used.